In a conventional rotary fuel pump, the angular position of a cam ring is adjusted by means of a servo advance arrangement. One known type of servo advance arrangement is described in our co-pending European patent application EP 1356196. The servo advance arrangement includes a main piston (referred to as the advance piston) which cooperates with a cam arrangement of the fuel pump to adjust the timing of fuel pump delivery. The advance piston is responsive to fuel pressure changes within an advance piston control chamber (the main advance control chamber). If pressure in the main advance control chamber increases, the advance piston is caused to move in a first direction so as to advance the timing of fuel delivery. If pressure in the main advance control chamber is reduced, the advance piston is caused to move in an opposite direction to retard the timing of fuel delivery.
A servo piston is operable in response to a transfer pressure signal (a signal dependent upon engine speed) to determine the position of the main advance piston. For certain positions of the servo piston, the main advance control chamber receives fuel at transfer pressure by means of a supply port provided in the advance piston and a flat machined on the outer surface of the advance piston. In such circumstances increased fuel pressure within the main advance control chamber causes the advance piston to move to advance engine timing. For other positions of the servo piston, a drain port is uncovered causing pressure within the main advance control chamber to decrease and, hence, the advance piston is moved to retard engine timing. The supply port and the drain port are aligned on a common axis on the advance piston so that both communicate with the flat, either to provide a flow route into the main advance control chamber through the supply port or a flow route out of the main advance control chamber through the drain port. The servo advance arrangement is also provided with a light load mechanism to permit the timing of fuel delivery by the pump to be varied when the engine operates under light load conditions.
In use, on sudden opening and closing of the throttle, the advance piston is caused to move rapidly between a no-load position and a full-load position (timing retardation), and then in a reverse direction between the full-load and no-load positions (timing advance). During advance, movement between the full-load and the no-load positions is rapid. Advance piston movement is also rapid during retardation between the no-load and full-load positions, although slightly less so compared with advance movement.
It has been observed that for certain applications the rapid movement of the advance piston during retardation results in a transient emission of undesirable white smoke. It is an object of the present invention to provide an improvement to the advance arrangement which removes or alleviates this problem.